With the Islamic holy month of Ramadan set to begin this weekend, dozens of Muslim World Cup players will have to make a tough call: which comes first — their religious beliefs or their athletic ambitions?

During Ramadan, healthy adults are expected to fast — meaning no food or drink — during daylight hours for 30 days. Most Muslims wake up before sunrise to eat and drink, then break their fast after sundown. Observing the fast is one of the five essential pillars of the faith.

In Brazil at this time of year, the sun rises between 6:30 and 7:00 a.m. Most World Cup games in the next rounds will be played in the early and late afternoons, and temperatures in some parts of the country have topped 80 degrees with high humidity.

All that means that Muslim athletes who choose to fast would be competing in muggy, hot, grueling conditions, having had no food or water for several hours, and without the ability to re-hydrate as they play.

Fasting? Have you seen the weather? I would die.

Germany, France, Belgium, and Switzerland all have Muslim players on their squads and have made it to the knockout stages of the World Cup. Algeria, a Muslim-majority country, and Nigeria, where half the country is Muslim, are also advancing.

Yaya Toure, a Muslim midfielder for the eliminated Ivory Coast team, was quoted in a United Arab Emirates newspaper as saying he would not consider fasting, if his team advanced.

Algerian players celebrate after the group H World Cup soccer match between Algeria and Russia at the Arena da Baixada in Curitiba, Brazil, on June 26. The match ended in a 1-1 draw, but Algeria qualified for the round of 16. The team is made up entirely of Muslim players. Ivan Sekretarev / AP

“Fasting? Have you seen the weather?" Toure told The National. "I would die."

The biggest concern, said Heather Gillespie, a sports medicine physician at the University of California Los Angeles, is dehydration. “That can lead to heat illness and heat stroke,” she said.

It can also cause tiny muscle tears that can last far longer than game day, said Kristen Gravani, a sports dietitian at the University of Washington, as well as cognitive problems.

“They won’t be able to focus,” she said.

If Ramadan is followed appropriately, there will be no reduction in the physical performances of players

Still, Jiri Dvorak, FIFA's chief medical officer, told a media briefing this week that players observing the fast should not suffer any deterioration in their physical condition.

"We have made extensive studies of players during Ramadan, and the conclusion was that if Ramadan is followed appropriately, there will be no reduction in the physical performances of players," Dvorak told reporters.

Gravani wasn’t aware of any research that examined players in the field, but she said that lab work has shown that if players maintain their diets at night they can perform well during some exercise routines.

There are also tricks to reducing body temperature, such as wrapping a cold towel around your neck. To control the dryness in their mouths, they can “mouth rinse” with water, she said.

Germany's Mesut Ozil said at a press conference on Wednesday that he would not be fasting. "Ramadan starts on Saturday, but I will not take part because I am working," he said at the conference. Thomas Eisenhuth / picture-alliance/dpa via AP

Though interpretations vary across Islam, consensus in the community seems to be that fasting during Ramadan should not be harmful to one's health. Already, there are exceptions for children, the elderly, pregnant women, and those who are ill or traveling.

Germany's Mesut Ozil said at a press conference this week that he had already made his decision.

"Ramadan starts on Saturday, but I will not take part because I am working," said Ozil.